ACS TRACE GAS DETECTION ATTEMPTS
A. Trokhimovskiy, A. A. Fedorova, O. I. Korablev, D. A. Belyaev, Space Research Institute of the Russian Academy of
Sciences (IKI RAS), Moscow, Russia (trokh@iki.rssi.ru), A. S. Braude, F. Montmessin, F. Lefèvre, LATMOS/IPSL, UVSQ
Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France, J. Alday, Open University, Milton Keynes,
U.K., Kevin S. Olsen, Department of Physics, University of Oxford,UK

Introduction: The ACS instrument onboard
ExoMars TGO has been developed to meet a variety
of science goals, in particular, the sensitive search for
trace species in the Martian atmosphere [1]. Regular
soundings in solar occultation mode started in March
2018. For methane ACS reported stringent 20 ppt
upper limits all over the planet [2,3], which are not
correlated with surface measurements by the MSL
rover [4]. With the methane lifetime of ~300 years,
conventional chemistry cannot explain the disagreement. ACS continues regular measurements to gain
statistics and in hope for possible methane spikes
reported previously [5]. The presence of methane
derivatives like ethane C2H6 is very unlikely, yet
ACS sets an improved upper limit of 50 ppt.
Recently discovered by ACS [6], hydrogen chloride (HCl) may be involved in many atmospheric
processes, so we looked for chloromethane (CH3Cl)
features in the data. The Viking landers detected
chloromethane during the search for organic compounds in Martian soils [7]; MSL again noted CH3Cl
during the pyrolysis of soil [8], still these detections
are believed to have a contamination source. The
ACS upper limit for CH3Cl in the atmosphere is better than 1 ppb, which is several times better than previous knowledge [9], and extends over seasons.
Sulfur species were searched as an indicator of
any volcanic outgassing from the surface of Mars.
For the most optimal sensitivity conditions, we determine upper limits of SO2 at 20 ppbv, H2S at 15
ppbv, and OCS at 0.4 ppbv [10]; the last value is
lower than any previous upper limits imposed on
OCS in the literature.
Nitrogen is known to be present in the Martian
atmosphere, forming the oxidation products derived
from ionospheric dissociation of N2. The first species to searched for with ACS is NO2. It has comparable model predicted abundances with NO in the
lower atmosphere. ACS is sensitive to NO2 with an
accuracy of 200 pptv matching model predicted concentrations [11]. Without a confident detection, the
median abundance over the entire ACS dataset
agrees with this value. In 2021 we started a dedicated
search campaign for NH3 and HCN. Preliminary
derived upper limits are 11 ppb and 1 ppb accordingly.

References
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ExoMars 2016 Trace Gas Orbiter. Space Sci. Rev.

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[10] Braude, A. S. et al. No detection of SO2,
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